Method for producing spheroidal graphite cast iron

ABSTRACT

A method for producing high-purity spheroidal graphite cast iron. The method comprises at least the steps of pretreating the melt in a closed converter and adding an inocculant when casting it into the mold. The solubility of the substances formed during the pretreatment and during the addition of the inocculant an be adjusted. The nucleation in the melt and the solidification in the mold can be brought into conformity by coordinating the pretreatment and the addition of inocculant.

[0001] The invention relates to a process for producing nodular castiron, at least comprising the process steps of pretreatment with puremagnesium in a converter and an addition of inoculant during castinginto the mold.

[0002] U.S. Pat. No. 5,098,651 has disclosed a process and a device forproducing nodular cast iron. In a converter, a treatment vessel for themolten cast iron, pure magnesium is introduced in a vessel compartmentwhich is accessible from the outside and is in contact with theremainder of the treatment vessel via openings in the partition betweenthe vessel compartment and the remaining treatment vessel. When theconverter is moved from a horizontal position, which has been adopted inorder to introduce the magnesium, into a vertical position, themagnesium is wetted by the molten material and dissolved, generatingreaction heat. The reaction with the pure magnesium acts in thefollowing way: inorganic compounds, comprising oxides, sulfates,silicates and other compounds with magnesium, are formed from thenonmetallic constituents of the melt. The melt is set in intensivemotion by the vapor pressure of the magnesium. The mineral compoundscoagulate, agglomerate and float, on account of the lower relativedensity, on the bath surface. The slags which are formed in this way canbe removed from the bath surface by skimming. This process isdistinguished, inter alia, by a bath composition with a high purity of,for example, less than 10 ppm of dissolved oxygen and a lowconcentration of nucleating agent of, for example, less than 100 ppm ofnonmetallic particles.

[0003] WO 99/45156 A1 describes a process in which, before the actualtreatment with magnesium, the molten cast iron is treated in successivesteps with calcium carbide for removal of oxygen and sulfur, withrolling scale to supply oxygen and with aluminum oxide or with calciumoxide to form calcium aluminates or calcium aluminum silicates. Theresult of this is that following the subsequent treatment withmagnesium, the cast iron includes nonmetallic inclusions which are in aplastic condition during the material-removing machining and thereforeserve as lubricants. Excessive quantities of inoculant are often addedin the subsequent process steps. Then, during casting into the mold,there is a risk of the quality being adversely affected because thereaction products lead to uncontrollable defects, such as slags, oxidesand undissolved inoculant particles. Uncontrollable conditions withregard to the nucleation state arise. Because the nucleation state isnot accurately known, it is impossible to accurately determine themetering of the inoculants. Lack of certainty about the nucleation stateoften leads to excessive quantities being metered in. This is expensiveand is not advantageous for the quality of the cast iron.

[0004] Working on the basis of this prior art, it is an object of theinvention to provide a process for producing nodular cast iron which atleast comprises the process steps of pretreatment with pure magnesium ina converter and addition of inoculant during casting into the mold, withthe minimum possible quantity of unnecessary oxides, slags or inoculantresidues being introduced into the melt and into the casting.

[0005] This object is achieved by a process for producing nodular castiron, at least comprising the process steps of pretreatment with puremagnesium in a converter and an addition of inoculant during castinginto the mold, in which process the solubility of the substances formedin the pretreatment and inoculation is adjustable.

[0006] Preferred refinements of the invention will emerge from thedependent claims.

[0007] It is advantageous for the minimum possible amount of impuritiesto be introduced in the cast iron for production of nodular cast iron.This is achieved by the fact that the pretreatment is carried out usinga mixture or a compound of pure metallic magnesium with one or moremetals from groups IIa and IIIa of the periodic system and with sulfur.This is also achieved by the fact that the pretreatment is carried outusing a mixture or a compound of pure metallic magnesium with sulfur andwith Ca, Ba, La or rare earths, the mixture or compound formingoxysulfides in the molten metal.

[0008] It is also advantageous for the total quantity of substancesadded to be kept as low as possible for the production of nodular castiron. This is achieved by the fact that the quantity of an inoculantbased on FeSi or CaSi which is added during casting into the mold issignificantly lower, preferably four times lower, than the amount of thecompounds or mixtures which form oxysulfides which is added in thepretreatment with magnesium.

[0009] The process proposed here is based on the known pretreatment withpure metallic magnesium in the closed converter. The process leads to animprovement in the quality of the nodular cast iron which is produced inthe magnesium converter, since casting defects caused by oxides andslags and further defects caused by inoculants are avoided. In existingfoundries with a magnesium converter, the process can be implementedeasily and without the need to add additional process steps. Theaddition of the compound or mixture which forms oxysulfides with thepure metallic magnesium results, in the pretreatment, both inpurification of the melt and in preconditioning of the nucleation statein a single process step. The chemical oxygen and sulfur activity in themelt can be set as accurately as possible, while the smallest possiblenumber of foreign bodies per unit weight of cast iron needs to be added.Since the oxysulfides which are formed have a relatively poor solubilityin the cast iron melt, the nuclei which are formed by reaction with themelt after the addition of the compound or mixture which formsoxysulfides have a sufficiently long life in the melt. It is possible totalk of a high nucleation capacity of the oxysulfides. The formation ofan oxysulfide in the pretreatment has shown that this has the leastpossible adverse effect on the final purity of the cast iron alloy and,at the same time, has a positive influence on the solubility of thenucleating substances.

[0010] The addition of the compound or mixture which forms oxysulfidesin the pretreatment leads to preconditioning of the melt, i.e. optimumpreparation of the melt for solidification in the mold. This optimumpreparation significantly reduces the demand for inoculant duringcasting. The amount and nature of the compound or mixture which isintroduced during the pretreatment in the magnesium converter can beselected in such a manner that it may even be possible to dispense withthe process step of adding inoculants during casting in the mold. Thelow level of foreign substances added has the minimum possible adverseeffect on the purity of the cast iron melt. Furthermore, the use ofoxysulfides has the following advantages: the relative density deviatesonly slightly from the relative density of the melt, so that it is easyto achieve a homogenous mixture. The grain size and grain shape can beselected in such a manner that the nuclei are retained for a longertime. Elements from groups IIa and IIIa of the periodic system, forexample Mg, Ca, Ba, La or rare earths, are advantageously selected asaddition to the magnesium in the pretreatment.

[0011] The subsequent casting from the casting furnace into the moldonly requires a small amount of inoculant to be added in order todefinitively set the state of nucleation. Since the oxysulfides whichhave been formed in the pretreatment have a higher nucleation capacitythan the inoculants which have been known hitherto, a small quantity,i.e. at most a quarter of the quantity which was added in thepretreatment, is sufficient to control the nucleation in the mold. Theinoculant which is added during casting into the mold can be used tocontrol and definitively set the rate, crystal shape and crystaldistribution during solidification. The inoculant used during castingis, for example, an alloy or compound which is based on FeSi or CaSiwith Ca, Ba, Mg, Au, Ge, La or rare earths.

[0012] The pretreatment with compounds or mixtures which formoxysulfides in combination with the magnesium treatment in theconverter, on the one hand, and the reduced addition of inoculant duringcasting, on the other hand, has the following further advantages: sinceonly small amounts of foreign bodies have been introduced, considerablyless slag is formed than in the conventional process, and consequentlyless iron is lost with the slag. Since overall less inoculant is fed tothe cast iron melt, fewer foreign bodies are included between thecrystals during solidification. As a result, the likelihood of castingdefects being formed is reduced. The fact that there are fewerinclusions of magnesium, silicon or oxygen compounds has a beneficialinfluence on the microporosity of the cast iron and promotes uniformsolidification in the mold. A lower microporosity is also advantageousfor the interaction of the molten material with the molding-materialmixture or with the sand from which the mold is formed. As a result, thesurface of the cast iron product is cleaner and more uniform, i.e. thereare fewer surface defects. This makes the surface of the cast-ironproduct easier to machine.

[0013]FIGS. 1 and 2 show a microscopic image of a nodular graphiteparticle with an undissolved core of an inoculant inside it. FIG. 2shows an enlarged view of the central area of the particle from FIG. 1.The nodular graphite particle was obtained from a melt which has beeninoculated with a compound of iron, silicon and lanthanum. The analysisof the core of the nodular graphite particle showed the followingcomposition: 41.7% of La, 47.2% of S, 9.1% of Mg and 1.7% of Si. Thisshows that the poor solubility of the compounds or mixtures which aresupplied in small quantities and form oxysulfides has a beneficialinfluence on the formation of nodular graphite particles and on thecasting properties.

[0014] The nodular cast iron products which are produced using thisprocess are distinguished by a high level of purity combined withrelatively low production costs. Less slag has to be disposed of orrecycled, and the amount of inoculants metered in can be considerablyreduced. The outlay on measuring and controlling the casting process, inparticular the outlay on equipment for analysis of the melt, can bereduced considerably because the process is more stable and morereproducible. The quantitative ratio of the foreign bodies added can bemodified between pretreatment and inoculant. The matching betweenpretreatment and addition of inoculant allows both the nucleation in themelt and the solidification in the mold to be adapted to one another. Anappropriate selection of the nature and amounts of the foreign bodiesmeans that under certain circumstances it is possible to dispense withthe step of adding inoculant during casting into the mold. Nodular castiron (SG iron) is distinguished by mechanical properties which are veryclose to the mechanical properties of cast steel.

1. A process for producing nodular cast iron, at least comprising theprocess steps of pretreatment with pure magnesium in a converter and anaddition of inoculant during casting into the mold, characterized inthat the solubility of the substances formed in the pretreatment and inthe addition of inoculant is adjustable.
 2. The process for producingnodular cast iron as claimed in claim 1, characterized in that thepretreatment is carried out using a mixture or a compound of puremetallic magnesium with one or more metals from groups IIa and IIIa ofthe periodic system and with sulfur.
 3. The process for producingnodular cast iron as claimed in claim 1 or 2, characterized in that thequantity of an inoculant based on FeSi or CaSi which is added duringcasting into the mold is significantly lower, preferably four timeslower, than the amount of the compounds or mixtures which formoxysulfides which is added in the pretreatment with magnesium.
 4. Theprocess for producing nodular cast iron as claimed in one of claims 1 to3, characterized in that the pretreatment is carried out using a mixtureor a compound of pure metallic magnesium with sulfur and with Ca, Ba, Laor rare earths, the mixture or compound forming oxysulfides by reactingwith the molten metal.
 5. The process for producing nodular cast iron asclaimed in one of claims 1 to 4, characterized in that during thesubsequent addition of inoculant, the inoculant based on FeSi or CaSicontains Mg, Ca, Ba, Au, Ge, Bi, La or rare earths.
 6. The process forproducing nodular cast iron as claimed in one of claims 1 to 5,characterized in that the oxysulfides formed have a grain size of 0.001to 0.03 mm.
 7. The process for producing nodular cast iron as claimed inone of claims 1 to 6, characterized in that the oxysulfides formed havea relative density of 4000 to 6000 kg/m³.
 8. The process for producingnodular cast iron as claimed in one of claims 1 to 7, characterized inthat the oxysulfides formed have a melting point of 1250 to 1550° C. 9.The process for producing nodular cast iron as claimed in one of claims1 to 8, characterized in that the specific number of nuclei of theoxysulfides formed is between 60 and 1200 mm⁻².
 10. The process forproducing nodular cast iron as claimed in one of claims 1 to 9,characterized in that the compounds or mixtures used for thepretreatment are introduced into the molten material in the converterseparately from the pure magnesium, joined or alloyed to the magnesiumor embedded in the magnesium as a wire, as granules or in the form of asandwich.
 11. The process for producing nodular cast iron as claimed inone of claims 1 to 10, characterized in that the amount and type ofinoculants introduced during casting into the mold are matched to theamount and type of the compounds or mixtures with magnesium and sulfurforming oxysulfides which are introduced during the pretreatment in theconverter.
 12. The process for producing nodular cast iron as claimed inone of claims 1 to 11, characterized in that the amount and type of thecompounds or mixtures introduced during the pretreatment in theconverter are selected in such a manner that there is no need to addinoculant during casting into the mold.